This invention relates to a defrost control system for a refrigerator and, more particularly, to a defrost control system that extends the period of time between defrost cycles during periods of moderate or light usage of the refrigerator.
Modern automatically-defrosting refrigerators aree designed to provide proper and efficient operation even during heavy usage conditions, and temperatures within the refrigerated compartment are maintained at levels appropriate for safe storage of food. The frequency of or, expressed alternatively, the interval between successive automatic defrosting operations is selected to prevent excessive accumulation of frost on the evaporator even under heavy usage conditions.
Various forms of variable defrost interval control systems have been developed. For example, adjustable defrost control timers have been provided which permit a user to optimize the defrosting interval for a particular ambient and usage conditions. Furthermore, various so-called "demand defrost" systems have been proposed whereby the refrigerator control system itself varies the interval between successive defrosting operations depending on various sensed parameters such as door openings and ambient humidity. The simplest and most commonly used demand defrost is achieved by connecting the motor in the defrost control timer such that it operates only when the refrigerator compressor is operating in response to a thermostatic temperature control. Thus, under heavy usage conditions, when the compressor runs frequently, the defrost control timer accumulates time at a faster rate. During low usage conditions, the compressor operates less frequently and the defrost control timer accumulates time at a slower rate. Such a defrost system, however, is inappropriate for extended periods of moderate or light usage of the refrigerator as there could be a defrost cycle initiated when in fact one may not be needed.
For example, during an extended vacation period when there are no door openings, the compressor will be still running periodically due to the continuing heat flow into the refrigerated storage space through the insulated walls of the enclosure. Consequently, the defrost control timer will continue to accumulate time, albeit at a slower rate, resulting in a defrost. As there is no moisture entering the insulated enclosure and no frost is accumulating on the evaporator, a defrost is unnecessary, and a waste of electrical energy.
By my invention there is provided a defrost control system for refrigerators which has a defrost control timer to initiate defrost, the timer is set for a predetermined period of compressor run time to initiate defrost, but in addition, a fixed time delay is incorporated before energizing the timer at the beginning of each compressor "on" cycle to extend the period of time between successive defrosts. The energy saving effect of this invention is most pronounced during light or moderate usage of the refrigerator, and diminishes with heavy usage.